A dual-polarization radiating element generally comprises two dipoles (or systems of dipoles) crossing one another at a 45° orthogonal polarization, one to generate the first polarization signal (−45°) and the other to generate the second polarization signal (+45°). Techniques for constructing radiating elements are varied.
The main conditions for a radiating element, as used in base stations' panel antennas, particularly include:    a) the radio performance of the radiating element (impedance, insulation between the two polarizations, radiation pattern) must be good and stable over a very broad frequency band,    b) the distribution surface area of the radio frequency current (RF) must be sufficient to allow the use of a small-sized reflector for the antenna, with the accompanying decrease in cost,    c) the structure for feeding the radiating element must be simple, such as a single coaxial cable for feeding each polarization of the radiating element,    d) the structure of the radiating element must preferentially enable the use of multiple radiating elements aligned along a common axis, in order to enable the integration of multiband antennas,    e) the radiating element must be as low-cost as possible (using small quantities of material, short assembly times, few parts, and moderate labor costs).
Several families of dual-polymerization radiating elements are already well known and used by manufacturers of different types of antennas. However, none of the existing radiating elements simultaneously and completely fulfills the five conditions described above.
A first family comprises coaxial radiating elements, each formed of two orthogonal half-wave dipoles. Provided that the shape of the dipoles is properly designed, the radio performance of these radiating elements is good. However, all of these radiating elements suffer from a limited surface area for distributing the RF current, which is only concentrated on the two orthogonal half-wave dipoles. Consequently, a wide reflector is necessary to achieve a given horizontal beamwidth on the antenna (65°, for example), which leads to additional costs on the antenna's structure (larger radome, etc.). This first family of radiating elements therefore does not meet condition (b) described above.
A second family comprises radiating elements, each formed of two half-wave dipoles separated by a distance of approximately one-half the wavelength at the operating frequency. The radio performance is good. The RF current's distribution surface area is wide, making it possible to obtain the desired antenna beamwidth with a limited-size reflector. However, the radiating elements must be fed at a four (two points for each polarization) leading to additional complexity and cost for the feeding network. This second family of radiating elements therefore does not meet conditions (c) and (e) described above. Some amount of surface area is available at the center of the radiating element such that it is possible to add a radiating element for multiband operation in order to satisfy condition (d).
There is an alternative radiating element that belongs to the second family. This radiating element has a sufficient surface area to distribute RF current, and it is fed only at two points (one point per polarization). The assembly time and cost of the material may be kept under control, particularly as a result of the milling technique. A major limitation of this type of radiating element is multiband integration. This is because adding radiating elements for a high frequency band requires using the technique of overlapping radiating elements. This means that the upper radiating element cannot use the shared reflector to generate its radiation pattern. The lower radiating elements are then used as reflectors, but their surface area is very low. This alternative from the second family of radiating elements only partially meets condition (d) described above.
A third family comprises dual-polarization radiating elements of the patch type (half-wave). The radio performance is not as good as for radiating elements formed of dipoles, in particular in terms of bandwidth, so condition (a) is only partially satisfied. This radiating element has a sufficient RF current distribution surface area, so that it can be used with a reflector whose dimensions are small. The feeding structure is simple because each dual-polarization radiating element can be fed with just two coaxial cables. The patch radiating element may be designed to have a low cost. It is possible to add another radiating element on top of the patch radiating element. In this situation, the added radiating element must be fed through the patch element, which is not easy. However, the upper radiating element cannot use the shared reflector to generate its radiation pattern, but rather must use the patch radiating element located below it as a reflector, with the drawback of a reduced surface area. This third family of radiating elements therefore only partially meets condition (d) described above.